In computer, information processing and control systems, it is necessary to store digital data and to retrieve it as desired. In a semiconductor memory, an array of storage or memory cells is used, with each memory cell holding one bit of data. When the information can be randomly put into or taken out of each memory cell or element as required, the array is called a random access memory (RAM) which may be static (SRAM) or dynamic (DRAM). The individual memory cells have data input and output lines, with each memory cell commonly having two output bit lines for indicating the presence of a 0 or 1 bit read out from the memory cell. The 0 and 1 bits are represented by different voltages which, when stored initially in the memory cells, may be quite close to each other and errors may accumulate tending to reduce the differences between the respective voltages to the order of tens of millivolts. Therefore, it is advantageous to include sense amplifiers connected to the output bit lines. These amplifiers are adapted to more accurately detect the voltages appearing on the bit lines and to latch the digital bit indicated thereby to provide a more accurate and faster readout.
One advantageous example of such a sense amplifier includes cross-coupled field affect pull down transistors, each having a first current carrying electrode (drain or source electrode) connected to a respective one of the bit lines and an associated pull up transistor, and a gate electrode connected to the other one of bit lines. Second current carrying electrodes (source or drain electrodes of the transistors) are connected together to receive a control signal which permits or prevents turning on of the transistors. If, for example, NMOS transistors are used, each transistor will turn on when the difference between the gate voltage and the source voltage is greater than the threshold voltage of the transistor. The signal to be sensed appears on only one of the bit lines which then carries a voltage higher or lower than the other bit line, depending on the value of the sensed signal. Consequently, when a control signal applied to the connected second current carrying electrodes is lowered to permit turn on of the two pull down transistors, a transistor having its gate electrode connected to the bit line carrying the higher voltage will turn on first. The other transistor will thereafter be maintained in its off state to latch the information read out from the memory cell.
The latching process of this type of sense amplifier having cross-coupled pull down transistors is accomplished by regenerative action. When the pull down transistor having its gate electrode connected to the bit line carrying the higher sensed voltage starts conducting, the other pull down transistor having its drain electrode connected to the bit line is off, the voltage on the bit line starts increasing gradually due to the pull up action of the pull up transistor associated with the pull down transistor in the off state. The increasing voltage on the bit line increases the voltage on the gate electrode of the conducting pull down transistor and causes it to conduct even more. This, in turn, causes the voltage on the drain electrode at the conducting pull down transistor, i.e., on the bit line carrying a lower voltage to decrease even more. However, this decrease is partially offset by the action of the pull up transistor associated with the conducting pull down transistor which attempts to pull the voltage up. To ensure that the low going bit line would decrease more and faster, the pull up transistor associated with the low going bit line should be smaller in size so that the current through it is small. However, a smaller pull up transistor associated with the high going bit line will increase the time it will reach its final high value. It is clear from the above discussion that of the two pull up transistors, only one serves a useful purpose, i.e., the one that pulls up the high going bit line. The other one plays a negative role by continuing to pull up the low going bit line.